The invention relates to the mixing apparatus and in particular to the impeller for blending of liquids and solids suspensions in liquids, for example where such liquids might contain a significant concentration of rag-like, stringy, fibrous material, and methods for forming the mixing apparatus.
Impellers for industrial mixing applications are widespread. Impeller efficiency for mixing can be characterized by the amount of energy that is needed for production of flow within a vessel. High efficiency of production of flow is important for many applications, including but not limited to flow sensitive mixing operations, such as blending of miscible liquids and solids suspension in liquids.
The benefits of high impeller efficiency typically include the potential energy saving during the operation and lower capital cost of the equipment due to low power consumption and subsequently small size.
Two parameters used in the calculation of impeller efficiency are Power Number (Np) and Flow Number (Nq). Both numbers typically are known as characteristic dimensionless constants of an impeller and can only be accurately determined experimentally. Pumping efficiency is traditionally expressed as flow per unit horsepower or more recently as the cube of the flow number divided by the power number (Nq3/Np).
High efficiency mixer pumping is typically achieved by utilizing marine propulsion propeller geometry which can include, but is not necessarily limited to, the geometric features known as helix, rake, camber, and skew. However the objective of a marine propeller, which is designed to produce thrust and generates flow as an unwanted byproduct, is diametrically opposite the objective of a propeller used in mixing, which is designed to produce flow that exhibits a specific velocity profile on the discharge side of the prop and where any thrust generated is an unwanted byproduct.
High pumping-efficiency propellers typically are created using a casting process where the geometry is machined into a mold using computer numerically controlled (CNC) milling machinery. CNC produced molds include those used to make metallic impellers and plastic impellers including fiberglass-reinforced-plastic (FRP). The process of casting or molding enables a manufacturer to design and produce virtually any configuration of impeller. In order to reduce impeller fabrication cost, particularly for large impellers since the cost of casting tooling increases exponentially with size, mixing impeller blades have, in the past, been fabricated from flat sheet metal stock, where rolls, bends, or combinations of rolls and bends, placed at strategic locations, have been used to simulate the more complex geometry found in molded propellers. These sorts of fabricated sheet metal impellers, that approximate higher efficiency helically based propellers, have been aggregately known in the mixing field as hydrofoils. Several hydrofoil impeller configurations are known, such as those of U.S. Pat. No. 5,297,938 (Von Essen et al), U.S. Pat. No. 4,468,130 (Weetman et al), and U.S. Pat. No. 5,052,892 (Fasano et al).
In some industrial applications, especially municipal waste water processing, rags (that is, stringy fibrous rag-like matter) collect on the impeller blades, causing loss of efficiency and shaft loading that can damage equipment, and often eventually requiring the mixer to be shut off for cleaning. The commercial embodiments of the 938, 130, and 892 hydrofoil patents have straight blades that are prone to collection of rags. Examples of mixing impellers that have been designed to resist the accumulation of rags, all of which are not hydrofoil impellers, include U.S. Pat. No. 1,850,199 (Bryant); U.S. Pat. No. 3,904,714 (Rooney et al); U.S. Pat. No. 4,163,631 (Connolly et al); U.S. Pat. No. 4,571,090 (Weetman et al); U.S. Pat. No. 4,575,256 (Armitage et al); and U.S. Pat. No. 7,473,025 (Howk). Examples of non-mixing impellers (also known as marine propellers) that have been designed to resist the accumulation of fibrous material include U.S. Pat. No. 4,482,298 (Hannon et al); and U.S. Pat. No. 5,249,993 (Martin); U.S. Pat. No. 4,163,631 (Connolly). An example of a mixing impeller that was not intentionally designed as rag-resistant but exhibits geometric properties that could provide some level of rag-resistance is U.S. Pat. No. 3,5142,1343 (Stoelting).
The current state of the art of rag-resistant mixing impellers do not embody high pumping efficiency geometry such as can be found in hydrofoil impellers or helically based propellers. Helically based marine propellers that have been designed to resist the accumulation of fibrous material are fabricated using casting technology and therefore require more expensive production tooling than is utilized in the fabrication of hydrofoil mixing impellers. Therefore, there is a need for an improved rag-resistant mixing impeller design that exhibits the higher pumping efficiencies found in helically based cast impellers, but that can also be made using the cost effective formed sheet metal fabrication techniques used in existing hydrofoil mixing impellers.